The official IrfanView is free for personal use. The concept of a "registration code" might stem from a few scenarios:
Here is the critical distinction most websites get wrong:
Since the software has been maintained by a single developer for over 25 years, many home users choose to "register" or donate even though it isn't required. This helps keep the project alive and free of bundled "crapware" or advertisements. official link
: For home use, educational purposes (schools, museums, libraries), or charity/humanitarian organizations, IrfanView is completely free and requires no registration.